1. Field of the Invention
The subject disclosure relates to systems for utilizing power generated by solar panels, and more particularly to an improved system for converting the power generated by a solar panel to improve safety and efficiency.
2. Background of the Related Art
In the modern world, the needs for electrical power are ubiquitous. However, many of the sources of electrical power such as nuclear energy and coal or fossil fuel power generation plants are not always feasible, and generate not only power but excessive pollution, exhaustion of resources and controversy. In an effort to avoid these drawbacks by utilizing the renewable energy of the sun, photovoltaic solar panel arrays are finding expanded use in the home environment and industry. Solar panel arrays are particularly well-suited to stand alone applications in isolated regions. Solar panel arrays not only function as an alternate energy source but excess power can be sold back to utility companies or stored for later use.
Referring to FIG. 1, a conventional home system is referred to generally by the reference numeral 10. The system 10 includes a solar panel array or solar array 20 mounted on a roof 22 and electrically connected to a control system 40. By mounting the solar array 20 on the roof 22, a maximum amount of sunlight, represented by arrows 24, without interference from trees, buildings or other obstructions is more likely. The control system 40 is typically stored within the basement of the house, and provides power to the load 26. The load 26 may also receive power from the utility grid 28 in a conventional manner.
The solar array 20 has a plurality of solar modules 30a-n which are comprised of a number of solar cells. Depending on the number of modules 30a-n, the system 10 can have a capacity from a few kilowatts to a hundred kilowatts or more. Typically, the number of modules is somewhat matched to meet the demands of the load because each module 30 represents a significant investment. Moreover, the roof 22 has limited area for conveniently and practically retaining the modules 30. Commonly, a module 30 consists of 36 photovoltaic cells which produce an open circuit voltage (OCV) of 21 to 23 VDC and a max power point voltage of 15 to 17 VDC. Standard power ratings for the solar modules 30 range from 50 W to 150 W. Thus, for an exemplary system 10, where two kilowatts are desired, as many as forty modules 30 may be needed.
For the most part, the prior art solar modules 30 are somewhat limited by their performance characteristics. In view of this, attempts have been made to optimize the solar module usage so that fewer solar modules 30 or less space are required. Tracking mechanisms have been added to actively orient each solar module 30 so that the incident sunlight is normal to the solar module 30 for increased efficiency. Other attempts at increasing efficiency and applicability of roof mounted solar arrays 20 have involved creating turrets to reduce the footprint thereof. Despite these attempts, solar arrays 20 are still uncommon and underutilized because of the additional expense and complexity these methods provide. As a result, the drawbacks of capacity and expense need to be overcome otherwise the range of practical applications for power from a solar arrays 20 will continue to be limited.
A common method for mounting a solar array 20 on the roof 22 is to mount each solar panel 30 individually and directly onto the surface of the roof 22. This method usually involves the installers carrying each solar panel 30 up to the roof and mounting them one at a time. Usually, the solar modules 30 are put into groups to form panels which, in turn, can be used to form the solar array 20. Solar modules 30 are live, i.e. outputting power, during installation. On a sunny day, the power generation can pose a safety hazard to the installers. There is a need, therefore, for an improved solar module control system which forces an improved module into a default off mode with no power output when not in use. Thus, adequate safety can be assured during installation and at other times of disconnection such as during replacement and repair.
Referring still to FIG. 1, an electrical conduit or conduits (not shown) carry the wiring that electrically interconnects the solar array 20 with the control system 40. The power output of each module 30 is carried individually to a string combiner 42 within the control system 40. Typically, the electrical conduit carries the outputs of the solar array 20 in a low voltage, high current bundle. As the size of the solar, array 20 increases, the thickness of the bundle and, in turn, the electrical conduit increases. As a result, the elect conduit is not only unsightly but represents significant danger if exposed. Thus, there is a need for a solar panel array which provides a high voltage and low current output that can be carried in relatively small cables which pose a minimal safety risk.
The control system 40 includes a central inverter 44 for changing the raw power from the string combiner 42 into usable power for the load 26. The central inverter 44 includes a matching DC/DC converter 46 and an AC/DC converter 48. An optional battery 50 is also shown disposed between the matching DC/DC converter 46 and the AC/DC converter 48 for use in an off grid system or as part of an uninterruptible power supply (hereinafter “UPS”). The matching converter 46 drops the raw solar array power down from the string combiner 42 to a desired level. When a battery 50 is used, the typical desired level of voltage is 54V. Thus, the power generated by the solar array 20 may be stored in the battery for use during the night or fed to the AC/DC converter 48 for use by the load 26 or sale via the utility grid 28. The AC/DC converter 48 receives the 54 VDC power and outputs an AC current at a desired voltage and frequency such as for example 120, 208 or 240 VAC at 50 or 60 Hz. Converters 46, 48 are well known to those of ordinary skill in the pertinent art and, therefore, not further described herein.
The matching converter 46 may also include maximum power point tracking (hereinafter “MPPT”) for varying the electrical operating point so that the solar array 20 delivers the maximum available power. This and other techniques for effectively using power generated by solar arrays are common. An example is illustrated in U.S. Pat. No. 6,046,919 to Madenokouji et al. and is incorporated herein by reference. From the foregoing, it may be observed that the MPPT optimizes the solar array 20 as a monolithic unit.
In actuality, the solar array 20 is made up of solar modules 30 that each typically includes thirty-two cells divided into two groups of sixteen. Each of the solar module cells and solar modules 30 may be from different manufacturers and have varied performance characteristics. Moreover, shading by clouds and the like varies the output from cell to cell and module 30 to module 30. Thus, significant improvements in the efficiencies of the solar panels 30 can be realized if each solar module 30 can be operated at peak power levels. Similarly, each group of sixteen cells or even each cell's performance can be enhanced by corresponding optimization. Such performance would permit solar arrays with less panels to reduce the cost of a given installation and broaden the range of practical applications for solar power. There is a need, therefore, for a cost effective and simple control system which can greatly increase efficiency in new and existing solar arrays.
Further, the typical solar array 20 has a variable output not only throughout the day but the output voltage also varies according to other parameters such as temperature. As a result, the central inverter 44 is also required to be a variable regulator to control these variations. In the United States, galvanic isolation is required for connection to the utility grid 28. The galvanic isolation is usually achieved by a 60 Hz transformer on the output of the central inverter 44. These prior art necessities further increase the cost and complexity of the control system 40. A solar array which does not need the central inverter 44 to act as a variable regulator or galvanic isolation would advantageously reduce the cost and complexity of the control system.
Further still, solar arrays 20 that are configured for grid connect only (without UPS function available) operate only while the utility grid 28 is on. In a problematic manner, if a utility grid outage occurs, the power generated by the solar array 20 cannot be accessed to run the load 26. Even for periodic outages lasting for only seconds, requirements are such that the solar array 20 cannot be reconnected for five minutes after the utility grid power has returned. Thus, a need exists for a solar array control unit which can supply power to the load even when a utility grid outage occurs.